Experimental studies in rodents demonstrated the carcinogenic potential of phenolphthalein, the active ingredient in some laxatives, administered at doses similar to the dose that could be used by humans. Ovarian cancer was one of the specific forms of cancer observed in these experiments, but few epidemiologic studies have examined ovarian cancer risk in relation to laxative use. Aims: Phenolphthalein, a constituent in many laxatives, showed clear evidence of carcinogenic activity in 2-year feeding studies in rodents recently completed by the National Toxicology Program (NTP). About 3% of the older U.S. population uses phenolphthalein-containing laxatives on a daily basis. We investigated the relation of phenolphthalein-containing laxative use with risk of ovarian cancer and adenomatous colorectal polyps in humans. We examined the association between epithelial ovarian cancer and use of phenolphthalein-containing laxatives in 356 epithelial ovarian cancer cases (256 invasive, 100 borderline) and 424 controls. Cases were identified through a population-based registry in Los Angeles County in 1992-1998, and controls were matched to cases by age, race/ethnicity, and neighborhood. Data on laxative use (specific brands, frequency of use, usual dose) were obtained by structured in-person interview. Compared to women who never used a laxative, ever use of a phenolphthalein-containing laxative was not associated with an increased risk of invasive ovarian cancer (odds ratio, OR, 1.1, 95% confidence interval, CI, 0.74, 1.5) or of borderline ovarian cancer (OR 0.74, 95% CI 0.37, 1.5). Total days used, mean number of pills per day, and cumulative dose were also unrelated to risk.